RF filters are used in many high frequency (RF and microwave) electronic applications. For example, in cellular telephone communications, users within each operating cell are assigned a unique operating frequency within the frequency band designated for cellular communications. Therefore, each time a cellular user places or receives a call, that call will be assigned to one of the allocated frequencies. The channel in the cell repeater station that is relaying the telephone call must be tuned to the specific frequency of the call.
A typical cellular communications frequency band spans 869 MHz-894 MHz, with channel frequencies spaced 630 kHz apart (Advanced Mobile Phone Service. (AMPS) frequency standard). The cellular telephone service provider will assign particular channel frequencies to different cell sites within its service area. For example, a typical cell site may have 24 channel frequencies assigned to it. Each of these channels has a repeater transmitter that operates at a channel frequency.
Typically, each channel in the cell station has a narrow bandpass RF filter at the RF output of the transmitter that must be tuned to the channel frequency. A narrow bandpass filter ensures that only signals on the frequency assigned to that channel are transmitted.
A conventional RF filter assembly employs tubular resonators secured in cavities of the filter housing. Decoupling rods may also be provided between resonators to provide an enhanced resonator decoupling effect. Decoupling rods allow the resonators to be placed closer together thereby reducing the size of the filter assembly.
The problem with making many conventional filter assemblies is that the tubular resonators and decoupling rods are typically machined and fabricated at substantial expense. Many are formed with an integral means for mounting the resonator or decoupling rod. Thus, filter assemblies having typical machined resonators and decouplers are very expensive to manufacture.
It would be an advantage to provide a resonator filter assembly which uses conventionally formed or drawn tubing as the resonators as well as decoupling rods. It would also be desirable to provide a standard, flexible method of mounting resonators and decoupling rods to the filter housing.